The present invention proposes techniques for creating multi-ink luminescent color halftone images with means of verifying their authenticity. In addition, the invented luminescent color halftone image synthesizing techniques enable increasing the attractiveness and aesthetics of color images.
Providing new authenticating techniques coupled with corresponding design tools is of high importance, since existing documents and valuable products are often counterfeited due to the availability of high-quality and low-priced color photocopiers, desktop scanners, color printers and image processing software.
These new authenticating and design capabilities rely on the extended gamut and the strong luminescent colors offered by the invented methods for creating color images with combinations of classical and of daylight luminescent inks. The presence of a luminescent image part, verified by using an illuminant different from a reference illuminant (e.g. a UV light source, a colored light source, or a white light source different from the reference illuminant) validates the authenticity of the document, respectively product. Security documents such as banknotes, checks, credit cards, identity cards, travel documents, business documents, diploma as well as medical drug packages, commercial art and fashion articles may benefit from the enhanced security and the new design features provided by color separations into classical non-luminescent and daylight luminescent inks.
A further application concerns valuable products where protective and decorative features can be combined. For example luxury goods such as watches and clocks, bottles of expensive liquids (perfumes, body care liquids, alcoholic drinks), clothes (e.g. dresses, skirts, blouses, jackets and pants), pieces of art (paintings) may show under normal daylight light striking luminescent color images and at the same time prevent counterfeits by making the unauthorized reproduction of such luminescent color images difficult to achieve with commercially available desktop scanners and printers.
The present invention relies on the synthesis of color images with daylight luminescent inks. Until now, daylight luminescent colors were mostly used within fluorescent markers and for painting purposes. Slightly fluorescent inks have been used in the past by the print industry for increasing the saturation of an ink, for example the magenta ink. Daylight luminescent inks produce, when seen under daylight, saturated and intense colors since part of the energy absorbed at excitation wavelengths within the UV and the visible wavelength ranges is reemitted by luminescence within the visible wavelength range. Since daylight luminescent inks absorb energy in the visible wavelength range, they cannot be additively mixed, such as for example fluorescent inks that are invisible under daylight and possibly create non-interfering emission colors under UV light. In addition, since for most daylight luminescent inks, energy is also absorbed in the UV excitation wavelength range, daylight luminescent inks form, thanks to their emission spectra, also visible colors, when seen under a UV illumination (also called “black light”).
The term “luminescence” encompasses both the de-excitation of molecules by fluorescent emission, where the lifetime of an exited molecule ranges between 10−10 s to 10−7 s and slower radiative emission de-excitation processes, where the lifetime of the excited state of molecules ranges between 10−7 s to several seconds. The terms “UV illumination”, “UV illuminant” or “UV light” mean that the illuminant emits within the UV wavelength range, between 150 nm and 400 nm. Commercially available UV lamps, also called blacklight lamps, have peaks at 254 nm (UVC wavelength range), 311 or 313 nm (UVB wavelength range) or at 365 nm (UVA wavelength range).
Throughout the present disclosure, we refer to the CIE-XYZ and the CIELAB colorimetric systems (see Color fundamentals for digital imaging, in Digital Color Imaging Handbook, Sharma G. Ed., Chap. 1, Sections 1.5 to 1.7, pp. 15-40, CRC Press, 2003, herein incorporated by reference, and denoted [Sharma 2003]). In the present disclosure, the L*, a*, b* coordinates, as well as the chroma C* and the hue h* of the CIELAB colorimetric system are written without the star, i.e. respectively L, a, b, C, h.
There were prior art attempts to create color images by using several luminescent inks each emitting in a different part of the visible wavelength range. U.S. Pat. No. 7,054,038, Method and apparatus for generating digital halftone images by multi color dithering, filed Jan. 4, 2000, to Ostromoukhov and Hersch (also inventor in the present patent application), teaches a multi-color dithering method where one or more inks are possibly fluorescent inks. However, since the teachings of that patent are limited to juxtaposed halftones, i.e. side by side printed surfaces of fluorescent and non-fluorescent inks, it cannot be applied to today's mainstream screening technologies, such as mutually rotated clustered dots, where ink dots do partially overlap. In addition, juxtaposed halftoning requires a high registration accuracy which is difficult or impossible to achieve on high-throughput and/or on large size printing systems (offset, gravure, flexo, serigraphy).
In several inventions, fluorescent inks which are invisible when seen under normal daylight are used to check the authenticity of documents under UV light. U.S. patent application Ser. No. 10/818,058, “Methods and ink compositions for invisibly printed security images having multiple authentication features”, to Coyle, W. J. and Smith, J. C, filed Apr. 5, 2004, proposes to create fluorescent color images with red, green and blue emitting fluorescent inks, which are invisible under day light. U.S. Pat. No. 7,005,166, “Method for fluorescent image formation, print produced thereby and thermal transfer sheet thereof,” to Narita and Eto (2002), teaches how to form an image with color gradations using fluorescent red, green and blue colorants, colorless under normal daylight and emitting fluorescence under UV illumination. U.S. patent application Ser. No. 10/482,892 “Halftone image produced by printing”, inventors Brehm L. and Erbar H., priority Jul. 25, 2001, also describes an additive fluorescent ink mixing process capable of creating continuous tone halftone images. However, since the proposed halftoning process is additive, it can only be applied to inks which to do not absorb in the visible wavelength range, i.e. to substantially invisible fluorescent inks.
U.S. patent application Ser. No. 11/785,931, “Printing color images visible under UV light on security documents and valuable articles”, filed 23 Apr. 2007 to Hersch (also inventor in present application), Donzé and Chosson (hereinafter: “Hersch, Donzé, Chosson 2007”) teaches a method for printing full color images invisible under daylight and visible under UV illumination with fluorescent inks which may have emission colors different from red, green and blue.
A fluorescent or luminescent optical symbol scanner is disclosed in U.S. Pat. No. 7,357,326, filed Nov. 30, 2005 to Hattersley and Blackwell III, relying on a light emitting diode illuminating in the visible spectrum at a first wavelength, and a filter and sensor for capturing light at a second wavelength. The presence of light at the second wavelength enables reading fluorescent or luminescent markings.
U.S. patent application Ser. No. 10/517,299, “System and methods for product and document authentication”, filed Jun. 7, 2002 to G. Jones II, S. Burke and P. McDonald describes a method of marking products by tags formed by luminescent inks having specific emission wavelength ranges and specific decay times.
U.S. Pat. No. 7,422,158, Fluorescent hidden indicium, filed Oct. 24, 2003 to Auslander and Cordery propose a print head system with a first ink having a first color under normal daylight and a second fluorescent ink having the same color as the first ink under normal daylight but discernible from the first ink when subjected to fluorescent-exciting radiation. This second ink visible only under a fluorescence exciting radiation enables creating covert markings. U.S. Pat. No. 7,379,205 “System and method for embedding and extracting key information” to J. D. Auslander et. al, filed 16 of Dec. 2003 adds the possibility of revealing key information by the second fluorescent ink under fluorescent exciting light.
U.S. Pat. No. 7,182,451 “Method and apparatus for halftone printing with multi-signal transmission ink” to J. D. Auslander, filed 24 of Oct. 2003, teaches a method for reducing the amount of a single printed fluorescent ink by using single ink halftoning without reducing the intensity of the emitted fluorescent signal in the same proportion.
U.S. Pat. No. 7,536,553 “Method and system for validating a security marking” to J. D. Auslander, R. A. Cordery and C. Zeller, filed 24 Apr. 2002, teaches a method for printing a security marking with an ink absorbing light under daylight (dark patterns) and emitting light under an excitation illumination. This security marking is viewed both under daylight and by fluorescence under fluorescent excitation illumination.
In contrast to the above mentioned disclosures, the present invention aims at creating variable intensity color halftone images visible under normal daylight which incorporate color halftones produced by combining classical non-luminescent and daylight luminescent inks. The colors of the individual daylight luminescent inks need not be the same as the ones of the classical inks. The synthesized color halftone images may represent continuous tone pictures from different origins such as photographs, graphic designs, paintings or synthetic images.